1. Field
This disclosure relates to novel sound attenuating structures, and in particular to locally resonant sonic materials (LRSM) that are able to provide a shield or sound barrier against a particular frequency range and which can be stacked together to act as a broad-frequency sound attenuation shield.
2. Background
In recent years, a new class of sonic materials has been discovered, based on the principle of structured local oscillators. Such materials can break the mass density law of sound attenuation, which states that in order to attenuate sound transmission to the same degree, the thickness, or mass per unit area, of the solid panel has to vary inversely with the sound frequency. Thus with the conventional sound attenuation materials low frequency sound attenuation can require very thick solid panels, or panels made with very high density material, such as lead.
The basic principles underlying this new class of materials, denoted as locally resonant sonic materials (LRSMs) have been published in Science, vol. 289, p. 1641-1828 (2000), and such materials are also described in U.S. Pat. No. 6,576,333, and U.S. Pat. No. 7,249,653 on the various designs for the implementation of this type of LRSM. Current designs still suffer from the fact that the breaking of the mass density law is only confined to a narrow frequency range. Thus in applications requiring sound attenuation over a broad frequency range, the LRSM can still be fairly thick and heavy.
Conventional means of blocking airborne sound usually requires blocking the air medium with a solid material. This has a disadvantage for noise blocking applications where air ventilation is also required.
U.S. Pat. No. 7,395,898 to Yang, et al., describes a sound attenuation panel comprising, a rigid frame divided into a plurality of individual cells, a sheet of a flexible and elastic material (membrane), and a plurality of weights (platelets). Each weight is fixed to the sheet of flexible material such that each cell is provided with a respective weight and the frequency of the sound attenuated can be controlled by suitably selecting the mass of the weight. In such sound attenuating structures, in the membrane-weight unit cells distributed on a planar panel are all substantially identical. In one type of system as described in U.S. Pat. No. 7,395,898, the membrane is typically rubber or another elastomer, and the weight has mass between 0.1 to 10 g.
U.S. Pat. No. 8,579,073 to Sheng, et al. describes an acoustic energy absorption metamaterial that includes at least one enclosed planar frame with an elastic membrane attached and has one or more rigid plates are attached to the membrane. The rigid plates have asymmetric shapes, with a substantially straight edge at the attachment to said elastic membrane, so that the rigid plate establishes a cell having a predetermined mass. Vibrational motions of the structure contain a number of resonant modes with tunable resonant frequencies.
In configuring resonant metamaterials, structures in which the membrane-weight unit cells distributed on a planar panel have been identical. Given a particular membrane material, e.g., rubber, the weight would have a defined mass. This results in a working frequency within a particular range as determined by the mass, moment of the displaced mass and Hooke's law.